Oxygen sensors customarily include electrodes applied to the solid electrolyte body, for example made of stabilized zirconium dioxide, to which electrodes are applied. In one form, one electrode is exposed to the gas to be measured, whereas the second electrode is applied to a surface of the solid electrolyte body which is exposed to a substance providing a reference oxygen concentration, for example to ambient air. To protect the solid electrolyte body, and the electrode exposed to the combustion gases, it has been proposed to apply a porous ceramic layer over the electrode and over the ceramic body at the side or surface exposed to the exhaust gases. In one form, the body is a closed tube. Reference is made to U.S. Pat. No. 4,021,326, Pollner et al, assigned to the assignee of the present application, describing such a sensor.
The electrodes of sensors of this type usually are made of platinum. It has also been proposed to make that electrode which is exposed to the gas to be measured, typically the exhaust gas from an internal combustion engine, of a platinum-rhodium alloy. Little attention has been given to the electrode which is exposed to the reference substance, typically ambient air, since that particular electrode is not of substantial importance for the operation or function of the sensor. It has been noted that the output from the sensors could still be improved, that is, that the conversion of the oxygen in the gas to be measured to an electrical signal could be better. This means that formation of the thermodynamic equilibrium of the gas at the electrode exposed to the gas to be measured is not achieved as well as would be desired. Consequently, the voltage difference which is being measured does not approach the value which is to be theoretically expected if the oxygen content of the gas to be sensed is low.